This invention relates generally to a multichannel record disc reproducing system and apparatus. More particularly, it relates to a system and apparatus for reproducing a multichannel record disc while suppressing the frequency band wherein the noise components are predominatingly distributed.
A discrete four-channel record disc system previously was proposed by one of the present applicants, Nobuaki Takahashi, and was patented in the United States as U.S. Pat. No. 3,686,471. A direct wave of a sum signal of a pair derived from two channels and an angle-modulated wave obtained by angle modulating a 30 KHz carrier wave responsive to the difference signal of a pair of two channels are multiplexed and recorded on the respective side wall of the disc sound groove.
In general, abnormal noises generated at the time of reproducing such a multichannel record disc can be classified broadly into the following two kinds, depending on the cause of the noise.
1. Abnormal noise is caused by wear upon the sound groove of the disc, whereby the carrier level of the angle-modulated wave drops greatly, and the noise level becomes higher than the carrier level.
2. Abnormal noise is caused by the nonlinearity of the mechanical systems of the cutter in the recording system, the pickup in the reproducing system, and the sound groove of the record disc, when the level of the direct wave sum signal is extremely high, particularly when the level of the high-frequency component is high.
The present applicants have previously proposed various systems for preventing the reproduction and generation of the above enumerated noises in demodulated signals. For example, if an angle-modulated wave is partially lacking, it is equivalent to deviation of the angle-modulated wave responsive to a low frequency. For this reason, one proposed system detects this frequency deviation and applies muting to shut off the signal. Another system detects the above mentioned deviation toward the low frequency, and, in response thereto, the loop gain of a phase-locked loop is decreased, thereby to constrict the lock range and prevent the phase-locked loop from locking to the noise component.
However, in accordance with these prior proposed systems, the suppressing of impulse noises (arising from causes such as scratches and dust in the disc sound groove) is effectively achieved with no great problems. However, if the level of a sum signal is high, particularly in a high frequency band, the angle-modulated wave signal is continuously disturbed. Then, the application of one of the above mentioned proposed systems gives rise to an attenuation or cutting off of the difference signal component, continuously for a certain time. High-fidelity reproducing cannot be carried out. Furthermore, these proposed systems are also accompanied by other problems such as unsatisfactory sound source localization.